Substrates like glass are used in a variety of applications such as architectural applications, automotive applications, aircraft applications, etc. Oftentimes, the substrates must be coated with a functional coating(s) to achieve the required performance properties. Examples of functional coatings include electroconductive coatings, photocatalytic coatings, thermal management coatings, hydrophilic coatings, etc.
A photocatalytic coating can be applied on, for example, a glass substrate, to keep the surface of the glass free of common organic surface contaminants. The photocatalytic coating works in the following manner: when the photocatalytic coating is exposed to ultraviolet radiation (“UV”), the coating absorbs UV photons and, in the presence of water or moisture, generates highly reactive hydroxyl radicals that tend to oxidize organic materials on the coated substrate. Ultimately, any organic material on the surface of the coated substrate gets converted to a more volatile, lower molecular weight material that can evaporate away or be washed away.
Titania (TiO2) is well known in the art as a material that has good photocatalytic properties. Conventionally, a TiO2 coating is polycrystalline with all of the crystallites that make up the coating having the same crystal structure. In a coating composition, TiO2 can be present in one of the following crystal structures: anatase, rutile and brooktite. Depending on the crystal structure of TiO2 present in the coating, the photocatalytic coating will exhibit different performance properties, i.e. photocatalytic activity, UV induced hydrophilicity, durability, etc. For example, a coating made up of TiO2 crystallites having the anatase crystal structure has a higher photocatalytic activity than a coating made up of TiO2 crystallites having the rutile or brooktite crystal structures.
Polycrystalline functional coatings like TiO2 coatings can be deposited on a substrate using a variety of techniques. For example, well known techniques such as spray pyrolysis, chemical vapor deposition (“CVD”) and magnetron sputtered vacuum deposition (“MSVD”) can be used to deposit a TiO2 coating on a glass substrate. The advantage of spray pyrolysis and CVD over other techniques for depositing a TiO2 coating on a glass substrate is they can be utilized on a float glass line during a glass production process. If the TiO2 coating is applied on-line during the glass making operation, the TiO2 coated glass can be produced at a reduced cost due to the efficiency of the process.
Regardless of the technique used to deposit a polycrystalline functional coating on a glass substrate, there is no process available at the current time for depositing a functional coating made up of a material that can be present in more than one crystal structure under fixed deposition conditions, wherein at least two crystal structures of the material that make up the functional coating are present at the exposed surface of the functional coating. For example, there is no process currently available for depositing a photocatalytic coating comprising TiO2 on a float glass line under standard operating conditions wherein at least two crystal structures of TiO2, for example, the anatase crystal structure and the rutile crystal structure, are present at the exposed surface of the TiO2 coating. Under conventional deposition processes, one specific crystal structure, for example, either the anatase crystal structure of TiO2 or the rutile crystal structure of TiO2 is present at the exposed surface of the coating depending on the deposition conditions.
The present invention provides a method for depositing a functional coating made up of one or more materials that can be present in more than one crystal structure over a substrate, wherein there are at least two crystal structures of the material that makes up the functional coating present at the exposed surface of the functional coating and the at least two crystal structures are deposited in a single operation or coating step. By controlling the types and amounts of crystal structures of the material that makes up the functional coating that are present at the exposed surface of the coating, a coated substrate having the most desirable properties for a particular application can be obtained.